Tire Pressure Monitoring Systems (TPMS) play an important role in vehicle safety and emissions reduction. Several countries and governing bodies have enacted mandatory regulations that require vehicles to have TPMS; for example U.S., European Union & Korea. A majority of this market is served by direct tire pressure monitoring systems, in which each tire contains a TPMS sensor module. Due to this high market penetration, the cost and the size of these sensor modules is of high importance. Current sensor modules consist of a tire pressure sensor (TPS) integrated circuit, a battery, antennas for communication, and a very small number of discrete passive electrical components.
Most sensor modules use a motion detection sensor to conserve the life of the sensor module's battery by entering power-down mode while the vehicle is parked. In this way, the service life of the sensor and its non-replaceable battery can be maximized. The typical motion detection sensor responds to g-force and is either an accelerometer or a shock sensor.
One requirement of the sensor module is to that of a bi-directional communication link. The uplink communication channel from the sensor module towards the TPMS receiver of an electronic control unit (ECU) is typically over an ultra-high frequency (UHF) radio link. The downlink communication channel towards the sensor is mainly used in during the production and testing of the sensor prior to installation in a tire and wheel assembly. TPMS sensors are able to receive data on the downlink from a low frequency (LF, typically 125 kHz) transmitter. Therefore, the typical TPS integrated circuit (IC) includes an LF receiver circuit. The LF receiver circuitry within the IC is connected to a resonant LF antenna network. This network typically consists of a capacitor, a resistor and a wire wound high sensitivity coil. The LF antenna circuitry is all contained within the confines of the sensor module.
The LF antenna coil is constrained by physics. It consists of a high permeability (μr) core, with many turns of fine-gage wire wrapped around it. The wire ends are terminated and the entire assembly is contained within a form that is compatible with printed circuit board (PCB) assembly equipment. The coil is delicate, costly, and somewhat immune to the size reduction that occurs with other passive electronic components. A typical LF antenna network costs about 0.15€ and occupies around 20 mm2 of PCB area. The possibility to further shrink the LF antenna is limited by physics. A smaller size antenna requires a higher sensitivity receiver. This, in turn, requires more operating current. Unless a higher communication frequency is chosen, the LF coil will likely remain the same size for the foreseeable future.
Furthermore, the LF antenna is resonant, in order to provide a voltage amplification of the very small (on the order of millivolts) voltage induced across the LF antenna coil. The resonant antenna network typically has a Q factor on the order of 5-10. One disadvantage of a resonant antenna is that magnetic impulse energy, e.g., from solenoids, relays or motors, will excite the resonant antenna and cause it to “ring” at its resonant frequency. These impulse noise events occur often in vehicles, and can disrupt LF communication from taking place.
There is a second type of uplink that has been used in the past with TPMS, that of a magnetic reed switch that, when actuated, initiates a “learn mode” in the TPMS sensor. Historically, this has been implemented using a mechanical switch. The advantage of this sort of uplink is that the only tool required is a permanent magnet. However, the magnetic reed switch has proven to be a reliability risk in terms of surviving the harsh mechanical environment that a TPMS sensor must endure. Furthermore, the size of the switch is significant (e.g., typically about 12 mm in length and about 2 mm in diameter). Like the LF antenna coil, the magnetic reed switch will likely remain the same size for the foreseeable future.
Therefore, an improved device that retains the LF uplink functionality required by the TPMS sensor module without incurring additional bulk and cost may be desirable.